Olga Podluzhnaya presents from 1:11 biphonation, a unique two-part singing technique. Actually, biphonation is a vocal disorder. It is assumed that the left and right vocal cords vibrate at different frequencies . But here it is an artistic intention and certainly not pathological.
I did a sound analysis of my favorite part at 1:30 to 1:36. There you can see unusual things: the two tones produced at the same time have a gap of 324 cents between the major and minor thirds. This interval does not occur in the overtone series. So it’s really two separate sounds. But they have far too many overtones. The expected overtones are laid over the spectrogram as a diagram. And these can also be seen in the spectrum. But there are still more overtones in between, which cannot come from the two fundamental tones. I don’t have an explanation. Anybody have an idea?